1. Field of the Invention
The present invention relates to mobile communication apparatus having a packet data retransmission control function, and more particularly, mobile communication apparatus performing packet composition processing at the time of retransmission control, for composing a retransmitted packet data with a packet data already received and stored.
2. Description of the Related Art
In order to cope with mobile multimedia services, such as Internet connection, high-speed data communication is required in the mobile communication system. Retransmission control is used as a means for efficiently transmitting packet data for high-speed data communication. The retransmission control (ARQ: Automatic Repeat Request) is a technique in which, when an error is detected in a packet received on a receiving side, a transmitting side is requested to transmit the packet of interest again.
Moreover, the hybrid ARQ (H-ARQ) has been put into practical use, in which an error correction code (FEC: Forward Error Correction) is combined with ARQ. Presently, a W-CDMA system is widely used as a third-generation (3G) radio communication system, and for example, H-ARQ is adopted in HSDPA (High-speed Downlink Packet Access) for increasing data communication speed (14 Mbps maximum) in the above W-CDMA.
In H-ARQ, error correction processing is performed prior to error detection on the receiving side, and when it is decided the reception is successful (no existence of error), ACK (normal decision) is transmitted to the transmitting side, while when the reception is decided to be unsuccessful (existence of error), NACK (abnormal decision) is transmitted to the transmitting side, so as to request to retransmit the packet data. The transmitting side then retransmits the packet data corresponding to the received NACK.
Further, in H-ARQ, packet composition processing is performed, in which the receiving side stores in a buffer a packet data having a detected error, so as to compose with the retransmitted packet data. By composing with the packet data stored in the buffer when the retransmitted packet data is received, reception quality is improved. As the number of times of retransmission increases, the degree of improvement increases, and a success rate of packet data reception increases.
FIG. 1 shows a diagram explaining the conventional retransmission control. The retransmission processing on the transmitting side and the receiving side of each mobile communication apparatus set is performed on the basis of a data length equal to a plurality (n>2) of TTIs (Transmission Time Intervals) (for example, 4 TTIs in FIG. 1), and decoding processing, reception decision processing, retransmission request (notification of ACK/NACK), etc. are performed on the basis of each data length of the transmitted data.
Now, let one radio frame (basic unit for transmitting/receiving packet data in each set of mobile communication apparatus) to be 1 TTI (Transmission Time Interval: transmission time of a minimum unit assigned to each user), then the retransmission processing is performed on the basis of each data length of n TTIs). Additionally, RTT (Round Trip Time) is defined to be a time from when the transmitting side transmits the packet data of the last TTI among the packet data ranging over the period of a plurality of TTIs, to when a reception decision result (ACK/NACK) is received from the receiving side. The RTT in FIG. 1 is 6 radio frames=6 TTIs.
In FIG. 1, a data of 4 TTIs is transmitted from the transmitting side. On the receiving side, whenever the data of each TTI is received, the received data is stored in a retransmission composition buffer on the basis of each TTI. For example, as shown in the figure, first, the receiving side stores a first TTI data #1-1 in the data #1 of 4 TTIs, and next, stores a second TTI data #1-2, and subsequently, stores a third TTI data #1-3, and finally, stores a fourth TTI data #1-4. In the TTI period next to the period of storing the fourth TTI data #1-4, the receiving side obtains a decoding parameter from the header (H in the figure) added to the first data #1-1, and performs decoding processing, error correction processing, and then reception decision, in regard to the data #1 for 4 TTIs. Further, in the next TTI, the receiving side notifies the transmitting side of the above decision result.
At this time, as shown in the figure, if an error is detected in the first TTI data #1-1, and NACK (reception failure) is reported as the reception decision result, the transmitting side retransmits data #1 of 4 TTIs, in response to the above NACK reception.
Each time the data of 1 TTI is received, the receiving side performs composition processing of the received data with the data already stored in the retransmission composition buffer, and stores the composed data into the retransmission composition buffer. By the above composition processing repeated four times, the retransmission data is stored as a composed data, and in regard to the above composed data, the decoding processing, the error correction processing and the reception decision processing are performed again. In the case shown in FIG. 1, when ACK (reception successful) is reported as a reception decision result, the transmitting side is shifted to transmit the next data #2.
Data retransmission control in the mobile communication has been described in the official gazette of the Japanese Unexamined Patent Publication No. 2005-151013, as an example.
Now, as described above, when one time data transmission/reception requires the plurality of TTIs (that is, the data length ranges over the plurality of TTIs), and when the retransmission control is performed by executing the reception decision after the data reception of the plurality of TTIs, the following problem occurs.
Namely, even when an error is detected in only one TTI data among the plurality of TTIs (in FIG. 1, error is detected only in the first data #1-1), it is necessary to retransmit the entire data of the plurality of TTIs, and thus, without exception, the data having no detected error is transmitted in duplication, which impedes effective use of radio resources.
Further, on the receiving side, because the reception decision processing is not performed until the entire data of the plurality of TTIs are received, the time from when the last TTI is transmitted to when the reception decision result is received on the transmitting side (i.e. RTT) becomes long. In other words, an idle time of no data transmission becomes long, and a long time is required for retransmission.